TWI718275B - Sintered mullite-containing body, its preparation method and composite substrate - Google Patents

Abstract

The present invention provides a mullite-containing sintered body, which contains at least one selected from the group consisting of silicon nitride, silicon oxynitride and SIALON in addition to mullite. The mullite-containing sintered body preferably has a thermal expansion coefficient of less than 4.3 ppm/°C at 40 to 400°C, a porosity of 0.5% or less, and an average crystal grain size of 1.5 μm or less.

Description

Sintered mullite-containing body, its preparation method and composite substrate

The invention relates to a mullite-containing sintered body, its preparation method and a composite substrate.

The mullite sintered body is generally made by sintering alumina (Al ₂ O ₃ ) and silicon dioxide (SiO ₂ ) at a ratio of 3 to 2 to become a material with good thermal shock resistance, with 3Al ₂ O ₃ . 2SiO ₂ said. As such a mullite sintered body, for example, as disclosed in Patent Document 1, a powder obtained by mixing 30% by mass of yttrium oxide stabilized zirconia (YSZ) powder with mullite powder is molded, and the molded body Those who do sintering. In Patent Document 1, a mullite substrate is cut out from a sintered mullite body, and the main surface of the mullite substrate is polished to become a base substrate that can be used for bonding with a GaN substrate. The coefficient of thermal expansion of GaN is 6.0 ppm/K in the range from room temperature to 1000°C, and the coefficient of thermal expansion of mullite is 5.2 ppm/K. Therefore, when considering the use of the two together, it is better to increase the thermal expansion coefficient of mullite to be close to that of the GaN substrate, so YSZ powder is mixed with mullite powder and sintered.

On the other hand, Patent Document 2 describes that a functional substrate made of lithium tantalate, lithium niobate, etc., and a support substrate made of mullite sintered system are directly bonded to a composite substrate to be applied to surface acoustic wave devices and other acoustic waves. Examples of devices. These sonic devices, due to the mullite as the supporting substrate The thermal expansion coefficient of the substrate is about 4.4ppm/°C (40~400°C), and the Young's modulus is 220GPa or more, which can reduce the expansion or contraction caused by the temperature change of the acoustic wave device itself, thereby greatly improving the frequency temperature Dependency. In order to directly bond the functional substrate and the supporting substrate, high flatness is required on each bonding surface. For example, it is described in Patent Document 2 that the average centerline thickness Ra is preferably 3 nm or less.

【Prior Technical Literature】

Patent literature

Patent Document 1: Japanese Patent No. 5585570

Patent Document 2: Japanese Patent No. 5861016

However, although Patent Document 1 describes a mullite sintered body in which a considerable amount of other components are added to the mullite to increase the coefficient of thermal expansion, Patent Document 2 describes a mullite with high purity The sintered body does not mention a mullite sintered body that lowers the coefficient of thermal expansion at all, let alone the fact that it is such a low thermal expansion mullite sintered body that the polished and refined surface has high surface flatness. In addition, even when a low thermal expansion and low rigidity mullite sintered body is used as a supporting substrate of a composite substrate, the composite substrate may curl due to a slight temperature difference.

The main purpose of the present invention is to solve these problems by making a mullite-containing sintered body have a lower thermal expansion coefficient and higher rigidity than a single mullite, and at the same time improve the flatness of the polished surface.

The mullite-containing sintered body of the present invention contains, in addition to mullite, at least one mullite-containing sintered body selected from the group consisting of silicon nitride, silicon oxynitride and Sialon Among them, the thermal expansion coefficient of 40~400℃ is less than 4.3ppm/℃, the porosity is 0.5% or less, and the average crystal grain size (average particle size of sintered particles) is 1.5μm or less. Compared with the mullite-containing sintered body, the mullite-containing sintered body has a lower thermal expansion coefficient and higher rigidity. In addition, the flatness of the polished surface can be improved.

The method for producing a mullite-containing sintered body of the present invention includes: (a) 50 to 90% by volume of mullite powder with an average particle size of 1.5 μm or less and 10 to silicon nitride powder with an average particle size of 1 μm or less 50% by volume, mixing so that the total is 100% by volume to obtain mixed raw material powder; and (b) forming the above-mentioned mixed raw material powder into a molded body of a predetermined shape by pressing the above-mentioned molded body at a pressure of 20~ 300kgf/cm ² , the sintering temperature is 1525~1700°C for hot pressing sintering to obtain a mullite-containing sintered body. This production method is suitable for producing the above-mentioned mullite-containing sintered body of the present invention. In addition, the average particle size of the powder is a value measured by a laser diffraction method (the same applies below).

The composite substrate of the present invention is a composite substrate in which a functional substrate and a support substrate are joined, and the support substrate is the above-mentioned mullite-containing sintered body. Since this composite substrate has a high flatness on the polished surface of the mullite-containing sintered body as a supporting substrate, it can be well bonded to a functional substrate. In addition, when the composite substrate is applied to a surface acoustic wave device, the frequency-temperature dependence is greatly improved. In addition, even in optical waveguide devices, LED devices, and switching devices, the performance is improved due to the small coefficient of thermal expansion of the supporting substrate.

10‧‧‧Composite substrate

12‧‧‧Piezoelectric substrate

14‧‧‧Support substrate

30‧‧‧Electronic device

32,34‧‧‧IDT electrode

36‧‧‧Reflective electrode

Figure 1 is a flow chart of the manufacturing steps of the mullite-containing sintered body.

FIG. 2 is a perspective view of the composite substrate 10.

FIG. 3 is a perspective view of the electronic device 30 manufactured using the composite substrate 10.

Hereinafter, the embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments. It should be understood and appropriately changed based on the general knowledge of the person in the field within the scope not departing from the gist of the present invention. Improvement etc.

The mullite-containing sintering system of this embodiment contains at least one selected from the group consisting of silicon nitride, silicon oxynitride, and sialon in addition to mullite. The mullite is preferably the most contained component (main component) in the sintered body, but it does not matter even if the component selected from the above group becomes the main component. The mullite-containing sintered body has a thermal expansion coefficient of less than 4.3 ppm/°C at 40 to 400°C, a porosity of 0.5% or less, and an average crystal grain size of 1.5 μm or less. This mullite-containing sintered body has a lower coefficient of thermal expansion and a higher Young's modulus (rigidity) than the mullite alone. In addition, since this mullite-containing sintered body has almost no pores with a porosity of 0.5% or less, and the average crystal grain size is as small as 1.5 μm or less, the flatness of the polished surface (polished surface) becomes high.

In the mullite-containing sintered body of the present embodiment, the number of pores having a maximum length of 1 μm or more per unit area of 100 μm×100 μm on the polished surface is preferably 10 or less. If the number of pores is 10 or less, the flatness of the polished surface becomes higher. The number of such pores is preferably 3 or less, and more preferably zero.

The mullite-containing sintered body of this embodiment preferably has a Young's modulus of 240 GPa or more, and a 4-point bending strength of 300 MPa or more. Silicon nitride or components derived from them have a higher Young's modulus or strength than mullite. By adjusting the addition ratio of silicon nitride to mullite, mullite-containing sintered bodies can be made The Young's modulus is more than 240GPa, and the 4-point bending strength is more than 300MPa. Furthermore, it is preferable that the 4-point bending strength is 320 MPa or more.

The mullite-containing sintered body of the present embodiment preferably has an average centerline thickness Ra of the polished surface of 1.5 nm or less. It is known to bond a functional substrate and a support substrate when used as a composite substrate in acoustic wave devices. However, using a mullite-containing sintered body with a polished surface Ra of 1.5 nm or less as a support substrate can be used as a support substrate. The bondability between the support substrate and the functional substrate becomes better. For example, the ratio of the actual joint area between the joint interfaces (joint area ratio) reaches 80% or more (preferably 90% or more). The centerline average thickness Ra of the polished surface is preferably 1.1 nm or less, and more preferably 1.0 nm or less.

The mullite-containing sintered body of this embodiment preferably has a thermal expansion coefficient of 40 to 400°C of 3.8 ppm/°C or less. In this way, a composite substrate containing the mullite sintered body as a supporting substrate is applied to an acoustic wave device When the temperature of the elastic wave device rises, the functional substrate has thermal expansion smaller than the original thermal expansion, which improves the frequency and temperature dependence of the elastic wave device. The thermal expansion coefficient of 40~400℃ is better than 3.5ppm/℃.

Next, an embodiment of the method for producing a mullite-containing sintered body of the present invention will be described. The manufacturing process of the mullite-containing sintered body is shown in Figure 1, including: (a) the step of preparing mixed raw material powder; (b) the step of manufacturing the mullite-containing sintered body.

‧Step (a): Prepare mixed raw material powder

The mixed raw material powder is prepared by mixing mullite powder and silicon nitride powder. As the raw material of mullite, it is better to use powder with high purity and small average particle size. The purity is preferably 99.0% or more, more preferably 99.5% or more, and more preferably 99.8% or more. The unit of purity is mass %. In addition, the average particle diameter (D50) is preferably 1.5 μm or less, and more preferably 0.1 to 1.5 μm. The raw material of mullite can be commercially available, or can be made of high-purity alumina or silica powder. As a method of producing a mullite raw material, for example, the method described in Patent Document 2 can be cited. As the silicon nitride raw material, it is better to use powder with a small average particle size. The average particle size is preferably 1 μm or less, and more preferably 0.1 to 1 μm. The mixing ratio of the mullite raw material and the silicon nitride raw material, for example, the mullite raw material is 50~90% by volume (preferably 70~90% by volume) and the silicon nitride raw material is 10~50% by volume (preferably (10-30% by volume) weighing in a total of 100% by volume, mixing with a mixer such as a ball mill, and drying with a spray dryer if necessary, to obtain a mixed raw material powder.

‧Step (b): Making mullite sintered body

The mixed raw material powder obtained in step (a) is molded into a molded body of a predetermined shape. The molding method is not particularly limited, and general molding methods can be used. For example, the mixed raw material powder can be directly press-formed by a mold. At the time of press molding, the mixed raw material powder is granulated by a spray dryer, and the moldability becomes good. In addition, an organic binder may be added to produce a green body for extrusion molding, or a slurry may be produced for plate-shaped molding. These procedures must remove the organic binder component before or during the firing step. In addition, high pressure forming can also be performed by cold isostatic press (CIP).

Next, the obtained molded body is fired to produce a mullite-containing sintered body. At this time, it is preferable to maintain the fineness of the sintered particles and to discharge the gas during sintering to improve the surface flatness of the mullite-containing sintered body. As its means, the hot pressing method is very effective. Because using this hot pressing method, compared with normal pressure sintering, it advances toward densification in the state of fine particles at low temperatures, and can suppress the remaining coarse pores that are common in normal pressure sintering. The firing temperature (maximum temperature) during this hot pressing is preferably 1525~1700°C. In addition, the pressing pressure during hot pressing is preferably ^{20 to 300 kgf/cm 2.} In particular, a low pressurizing pressure is preferable because the hot-pressing jig can be miniaturized or have a long life. For the maintenance time of the firing temperature, an appropriate and appropriate time can be selected in consideration of the shape or size of the molded body, the characteristics of the heating furnace, and the like. The specific preferred maintenance time is, for example, 1 to 12 hours, more preferably 2 to 8 hours. The firing atmosphere is not particularly limited, and the atmosphere during hot pressing is generally an inert atmosphere such as nitrogen and argon. The temperature increase rate or the temperature decrease rate may be appropriately set in consideration of the shape or size of the molded body, the characteristics of the heating furnace, and the like. For example, it may be set in the range of 50 to 300° C./hour.

Next, an embodiment of the composite substrate of the present invention will be described. The composite substrate of the present embodiment is formed by joining a functional substrate and the above-mentioned mullite-containing sintered support substrate. This composite substrate exhibits good bonding properties because the ratio of the bonding area of the two substrates becomes larger. The functional substrate is not particularly limited, and examples thereof include lithium tantalate, lithium niobate, gallium nitride, and silicon. The bonding method is preferably direct bonding. In the case of direct bonding, the respective bonding surfaces of the functional substrate and the supporting substrate are activated after being polished, and the two substrates are squeezed in a state where the two bonding surfaces are aligned face-to-face. The activation of the bonding surface includes, for example, irradiation of an inert gas (argon, etc.) ion beam to the bonding surface, as well as plasma or neutral atom beam irradiation. The thickness ratio of the functional substrate and the supporting substrate (thickness of the functional substrate/thickness of the supporting substrate) is preferably 0.1 or less. Figure 2 shows an example of a composite substrate. Composite substrate 10 The piezoelectric substrate 12 and the supporting substrate 14 which are functional substrates are bonded by direct bonding.

The composite substrate of this embodiment can be applied to electronic devices and the like. As such electronic devices, in addition to acoustic wave devices (surface acoustic wave devices or Lamb wave elements, thin film resonators (FBAR), etc.), LED devices, optical waveguide devices, switching devices, etc. can also be cited. When the above-mentioned composite substrate is used in an acoustic wave device, since the thermal expansion coefficient of the mullite-containing sintered body as the supporting substrate is as small as less than 4.3 ppm/K (40 to 400°C), the frequency and temperature dependence is greatly improved. FIG. 3 shows an example of the electronic device 30 manufactured using the composite substrate 10. The electronic device 30 is a 1-port SAW resonator, that is, a surface acoustic wave device. First, the piezoelectric substrate 12 of the composite substrate 10 uses general photolithography technology to form a pattern of a plurality of electronic devices 30, and then, the electronic devices 30 are cut out one by one by cutting. The electronic device 30 uses the photolithography technology to form IDT (Interdigital Transducer) electrodes 32 and 34 and a reflective electrode 36 on the surface of the piezoelectric substrate 12.

In addition, the present invention is not limited to any of the above-mentioned embodiments, and various aspects can be implemented within the technical scope of the present invention.

1. Making mixed raw material powder

5mm的氧化鋁圓石以球磨機混合，藉由噴霧乾燥機製作混合原料粉末。 As the raw material for mullite, commercially available mullite powder with a purity of 99.9% or more and an average particle size of 1.5μm is used. As the raw material for silicon nitride, a commercially available silicon nitride with a purity of 97% or more and an average particle size of 0.8μm is used. powder. Weigh the mullite raw material and silicon nitride raw material in the ratio shown in Experimental Examples 1 to 3 in Table 1, and use

5mm alumina pebbles are mixed with a ball mill, and mixed raw material powders are produced by a spray dryer.

2. Production of mullite-containing sintered body.

The mixed raw material powders of Experimental Examples 1 to 3 were poured into a mold with a diameter of about 125 mm, and formed in a disk shape with a thickness of about 10 to 15 mm at ^{a pressure of 200 kgf/cm 2} to obtain a mullite-containing molded body. Next, the mullite-containing formed body is housed in a graphite mold for hot pressing with an inner diameter of about 125 mm, and a mullite-containing sintered body with a diameter of about 125 mm and a thickness of about 5 to 8 mm is produced in a hot pressing furnace. In addition, the maximum temperature (calcination temperature) at the time of firing was 1650°C, the maintenance time at the firing temperature was 5 hours, and the rate of temperature increase and the rate of temperature decrease were both 100°C/hour. Press load in the above temperature increase of 900 deg.] C reaches 200kgf / cm ^2, the furnace atmosphere was up to 900 deg.] C under vacuum, after reaching 900 ℃ introducing N _2, sintered under N _2. After the firing temperature is maintained for a predetermined time, the temperature is lowered to 1200°C, the control of the pressurization load and the ambient gas in the furnace is stopped, and the furnace is naturally cooled to room temperature. In addition, in Experimental Example 4, a compact was similarly formed using only mullite powder to produce a sintered body.

3. Characteristic evaluation

Test pieces (4×3×40 mm size anti-folding rods, etc.) were cut out from the sintered bodies of Experimental Examples 1 to 4, and various characteristics were evaluated. In addition, the polished surface of the sintered body is one obtained by polishing one side of a test piece of about 4×3×10 mm to refine it into a mirror-like surface. Grinding is carried out sequentially with 3μm coarse diamond grains and 0.5μm coarse diamond grains, and the final refinement uses diamond coarse grains below 0.1μm for polishing and polishing. The evaluated characteristics are as follows.

(1) Crystallinity

The sintered body is crushed, and the crystal phase is identified by an X-ray diffraction device. The measurement conditions are CuKα, 50kV, 300mA, 2θ=5-70°, using a rotating target cathode Polar X-ray diffraction device (Riji Electric Mechanism RINT).

(2) Crystallization ratio

From the X-ray diffraction analysis of (1) above, the peak area ratio of each crystal phase was calculated. The peak area of the mullite (210) plane (2θ=26.2°) was regarded as 1, and the peak area of each crystal phase relative to this was regarded as the crystal phase ratio. Here, as the representative peaks of each crystal phase, silicon nitride uses (101) plane (2θ=20.6°), Sialon uses (3-20) plane (2θ=24.6°) of _{Si 2} Al ₃ O _{7 N and} The (200) plane of Si ₅ AlON ₇ (2θ=26.9°).

(3) Bulk density and porosity

Using a flexural rod, the bulk density and porosity were measured by the Archimedes method using pure water.

(4) Young's modulus

According to JIS R1602, it is measured by static bending method. The shape of the test piece is a 3mm×4mm×40mm anti-folding rod.

(5) Flexural strength

According to JIS R1601 as a guideline, the 4-point flexural strength was measured. The shape of the test piece is a 3mm×4mm×40mm anti-folding rod or half of the size.

(6) Thermal expansion coefficient (40~400℃)

According to JIS R1618 as the criterion, it is measured by push rod differential method. The shape of the test piece is 3mm×4mm×20mm.

(7) Number of stomata

As described above, the polished surface of the refined sintered body was observed with SEM, and the number of pores existing per 100 μm×100 μm with a maximum length of 1 μm or more was counted.

(8) Surface flatness (Ra)

For the polished surface of the refined sintered body as described above, AFM was used to measure the centerline average roughness Ra. The measurement range is 10μm×10μm.

(9) Average particle size of sintered particles

The polished surface of the refined sintered body as described above is chemically etched with phosphoric acid, the size of 200 or more sintered particles is measured by SEM, and the average particle size is calculated using linear analysis. The coefficient of linear analysis is 1.5, and the value obtained by multiplying the length measured by SEM by 1.5 is used as the average particle size.

(10) Joinability

Discs with a diameter of 100 mm and a thickness of about 600 μm were cut out from the sintered bodies of Experimental Examples 1 to 4. After the disc is polished and refined as described above, it is washed to remove particles or pollutants on the surface. Next, this disc is used as a supporting substrate, and the supporting substrate and the functional substrate are directly bonded to obtain a composite substrate. In other words, firstly, the respective bonding surfaces of the supporting substrate and the functional substrate are activated with an argon ion beam, and then the two bonding surfaces are bonded facing each other and pressed at 10 tonf to obtain a bonded composite substrate. As the functional substrate, a lithium niobate (LN) substrate was used. In the evaluation of the joint property, judged from the IR penetration image, the joint area ratio is more than 90% as "best", the ratio of more than 80% and less than 90% is "good", and the ratio of less than 80% is "not good".

4. Evaluation results

The mullite-containing sintering system of Experimental Examples 1 to 3 is formed by sintering the mixed raw material powder of mullite raw material and silicon nitride raw material, and a part of the silicon nitride changes into sialon due to the firing . Since the mullite-containing sintered bodies of Experimental Examples 1 to 3 contain silicon nitride, etc., compared to the sintered body containing only mullite of Experimental Example 4, the Young's modulus and 4-point bending strength are improved. That is, the Young’s modulus increases Above 240GPa, the 4-point bending strength is increased to more than 320MPa. In addition, the mullite-containing sintered bodies of Experimental Examples 1 to 3 have a thermal expansion coefficient of less than 4.3 ppm/°C (3.5-4.1 ppm/°C) at 40 to 400°C, which is compared with the mullite-containing sintered body of Experimental Example 4. The sintered body has a lower value. Furthermore, since the mullite-containing sintered body of Experimental Examples 1 to 3 or the mullite-containing sintered body of Experimental Example 4 has a porosity of 0.5% or less (less than 0.1%), the average crystal grain size is 1.5μm or less (1.0~1.2μm), the average roughness Ra of the center of the polished surface is as small as 1.1nm or less (0.9~1.1nm). Therefore, when the discs cut out from the sintered bodies of Experimental Examples 2 to 4 are directly bonded to a functional substrate, the bonding area ratio of any one of them is "best", which is 90% or more. When the disc cut out of the sintered body is directly bonded to the functional substrate, the bonding area ratio is 80% or more but not 90% "good". In addition, since the center average roughness Ra of the polished surface reaches such a small value, the number of pores provided is 3 or less (zero).

In addition, Experimental Examples 1 to 3 correspond to Examples of the present invention, and Experimental Example 4 corresponds to Comparative Examples. The present invention is not limited to these experimental examples.

This application is based on claiming the priority of Japanese Patent Application No. 2016-058970 for which it filed on March 23, 2016, and its contents are all included in this specification by reference.

Claims (6)

A mullite-containing sintered body, which contains at least one mullite-containing sintered body selected from the group consisting of silicon nitride and SIALON in addition to mullite. Among them, 40 The thermal expansion coefficient at ~400℃ is less than 4.3ppm/℃, the porosity is 0.5% or less, the average crystal grain size is 1.5μm or less, the 4-point bending strength is 320~350MPa, and the average thickness of the center line of the grinding surface Ra is 1.5 nm or less, the crystal phase ratio of silicon nitride is 0.01 to 0.30, and the crystal phase ratio of Sialon is 0.57 to 3.24. According to the mullite-containing sintered body according to the first item of the scope of patent application, the number of pores with a maximum length of 1 μm or more per unit area of 100 μm×100 μm on the polished surface is 10 or less. The mullite-containing sintered body according to item 1 or 2 of the scope of patent application has a Young's modulus of 240 GPa or more. According to item 1 or 2 of the scope of patent application, the mullite-containing sintered body has a coefficient of thermal expansion between 40 and 400°C of 4.1 ppm/°C or less. A manufacturing method of a mullite-containing sintered body includes the following steps: (a) 50-90% by volume of mullite powder with an average particle size of 1.5 μm or less and silicon nitride powder with an average particle size of 1 μm or less 10 The step of mixing ~50% by volume to a total of 100% by volume to obtain mixed raw material powder; and (b) forming the above-mentioned mixed raw material powder into a molded body of a predetermined shape by pressing the above-mentioned molded body under a pressure of 20~300kgf/cm ^{2. The} sintering temperature is 1525~1700°C for hot pressing and sintering to obtain a mullite-containing sintered body, wherein the heating rate during firing is 50~300°C/hour, and the maintaining time of the firing temperature is 2 ~8 hours. A composite substrate is a composite substrate that joins a functional substrate and a support substrate. The support substrate is a mullite-containing sintered body according to any one of the first to fourth patent applications.

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